Solid-state emergency power supply

ABSTRACT

An emergency power supply is described comprising a switch and a low-level dropout circuit. The switch comprises a power transistor which provides power to the load from the battery and is driven by an amplifier. The low-level dropout supply circuit is a complementary bistable multivibrator which turns the switch off in going from a &#39;&#39;&#39;&#39;conductive&#39;&#39;&#39;&#39; state to a &#39;&#39;&#39;&#39;nonconductive&#39;&#39;&#39;&#39; state when the battery voltage drops to a predetermined level. There is also provided a delay-on timer which keeps the emergency lamp on for a selected period of time after external power restoration. In addition, a solid-state charger is provided to maintain a properly charged battery.

United States Patent I William .I. Roddi 3,387,14] 6/1968 Howald PrimaryExaminer-Robert K. Schaefer Assistant Examiner-H. J. HohauserArtorneys-Alfred J. Snyder, Jr., Robert H. Robinson,

Raymond L. Balfour and Joseph M. Corr ABSTRACT: An emergency powersupply is described comprising a switch and a low-level dropout circuit.The switch comprises a power transistor which provides power to the loadfrom the battery and is driven by an amplifier. The low-level dropoutsupply circuit is a complementary bistable multivibrator which turns theswitch off in going from a conductive state to a nonconductive" statewhen the battery voltage drops to a predetermined level. There is alsoprovided a delayon timer which keeps the emergency lamp on for aselected period of time after external power restoration. In addition, asolid-state charger is provided to maintain a properly charged battery.

PATENTED JUL27 I971 sum 2 or 2 INVENTOR William J. Roddi SOLID-STATEEMERGENCY POWER SUPPLY BACKGROUND OF THE INVENTION Emergency supplyunits having storage batteries as their sources of energy have been usedfor supplying electrical energy during periods of power failure as aresult of storms or fires or other causes. One such unit is an emergencylighting system which provides light for hallways and other passagewaysin buildings when there has been a loss of power supply and the buildinglights are no longer working.

A conventional circuit for such power supplies uses a sealed mercuryrelay switch whereby a coil is energized by the AC power and holds acylinder in position above a pool of mercury. Upon failure of the ACsource, the coil is no longer energized and the cylinder drops into thepool of mercury causing the level of mercury to rise and make contact tocomplete the circuit to the battery to turn on the emergency light. Somedisadvantages of such a circuit include the fact that the mercury switchis expensive, takes up space and a vibration noise occurs when thecylinder is in the pool of mercury.-

The conventional circuit does not have a means for deenergizing thecircuit when the potential of the battery drops to a low level. Instead,the conventional circuit permits the battery to discharge below a properdepth and eventually insufficient energy is being supplied to the lampto light it with any degree of illumination. Also, a continuedoverdischarge can do great harm to the battery.

This invention provides a fully solid-state circuit which overcomes theabove disadvantages of the conventional circuit and provides a reliablemeans for supplying power in cases of emergency.

SUMMARY OF THE INVENTION The primary purpose of this invention is toprovide a solidstate circuit for power supply systems such as emergencylighting systems wherein the circuit is inexpensive, compact, andreliable in operation.

This invention provides a switch comprising three transistors, two ofwhich are connected to each other as a darlington amplifier which iscoupled to a third power transistor which is connected between a batteryand a load. The circuit of this invention further achieves the object ofhaving a means for automatically turning off the emergency power supplywhen the battery voltage has dropped to a certain level and therebyprotect the battery from being discharged too deeply. By automaticallyturning off the emergency power supply when the battery voltage hasdropped to a certain level, the circuit of the invention assures thatthe power loss from the battery is not too great and that recharging canbe accomplished over relatively short periods of time.

It is an additional object of this invention to provide a circuit for anemergency lighting system in which the lamp is automatically turned onwhen the external power supply fails and is automatically turned offwhen the external power supply is restored or when the battery potentialhas dropped to a certain value. This circuit provides a low-leveldropout feature wherein once the battery potential decreases a certainamount, the lamp will not go on again until the circuit has been resetto a conductive state by the external source of power. It is an objectof the invention to provide that the dropout circuit will not be resetby the battery alone but by the external power source only.

It is another object of this invention to provide a third feature whichcan readily be incorporated into the emergency power supply circuit ofthis invention. This third feature is a delay on timer circuit whichdelays the signal for turning off the circuit from appearing at theinput of the switch for a period of time after restoration of externalpower. This subcircuit therefore is useful in those applications such asemergency lighting circuits wherein it is desired to keep the emergencylights on for a period of time while the main lights, such as mercuryvapor lights, are being energized by the external power supply.

A solid-state charger is also provided for accurately monitoring thebattery charge and for maintaining the battery at the proper chargelevel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of one formofthe circuit of this invention.

FIG. 2 is a schematic of an alternate form of the circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention provides areliable solid-state circuit for use in emergency power supply unitssuch as emergency lighting systems. The circuit basically comprises aswitch to turn the lamp on and of? together with a bistablemultivibrator lowlevel drop out circuit which causes the lamp to turnoff when the battery potential has dropped to a predetermined level. Analternate form of the circuit includes a delay-on timer which is made upof solid-state components and which keeps the lamp on for a period oftime after restoration of the external power supply. This delay on timeris useful in those situations where a period of time is required to turnon the building lights after restoration of power. For example, in thoseinstances having mercury vapor lights, which require a warmup period,this delay-on timer feature of this circuit can be useful. In both formsof the circuit, there is provided a solid-state charger for charging thebattery. Although the following discussion will specifically relate toan emergency lighting system it is not intended to limit the inventionto such applications since it can be used in a variety of emergencypower systems.

Turning to FIG. I, there is shown a preferred embodiment of the circuitof this invention. A stepdown transformer 20 has a primary winding 21connected to an AC power source and a secondary winding 22 having a pairof output terminals and a center tap. The output of the secondarytransformer is rectified through diodes 23 and 24 which have a commonoutput line 25. A filtering capacitor is shown at 27. A battery 30 isshown having its positive terminal connected to common bus line 31 andits negative terminal connected to the other common conductor 32. Thecharge controller which continually monitors and charges the battery isshown as 35 and is connected across the battery and to the AC powersource.

As noted previously, the invention comprises a basic switch circuit,shown generally at 36, combined with a low-level dropout circuit,generally designated as 37, wherein the basic switch circuit comprisesan amplifier coupled to a power transistor for supplying current to thelamp. Therefore, there is shown a PNP power transistor 38 having anemitter, base and collector. The emitter of this transistor is connectedto the common busline 31 while the collector is connected to the othercommon conductor 32 through the load 39 which is connected to outputterminals 39a and 3912 which in this instance is a lamp. The base of thetransistor 38 is connected to a common point 40 to which is connected tothe emitter of a PNP transistor 41. A resistor 42 is connected betweenthe common point 40, and the busline 31. The collector of transistor 41is connected to the other conductor line 32 through resistor 43.Transistor 41 is coupled to a third PNP transistor 44 wherein the baseof transistor 41 is directly connected to the emitter of transistor 44and the collector of transistor 44 is connected to the collector ortransistor 41 at the junction of point 45. The transistors 41 and 44therefore form an amplifier of the darlington type. The emitter oftransistor 44 is connected to the common busline 31 through a resistor46. The base of transistor 44 is tied to juncture joint 47 betweenresistors 48 and 49. Resistor 48 in turn is connected to the rectifieroutput line 25 while resistor 49 is connected to the collector oftransistor 51. This latter transistor forms part of the dropout circuitand will be discussed subsequently.

For the moment we will forget the presence of the low-level dropoutcircuit and assume that transistor 51 is conducting so that resistor 49is shorted to common line 32, and describe the operation of the switch.This description will begin at the point transistor to conduct. Whentransistor 44-conducts, this acts to bias the base of transistor 41 andthat transistor also conducts because transistors 44 and 41 areconnected in darlington amplifier fashion.

Because of this conduction and low loss across the transistor pair of 41and 44 resistor 43 is effectively connected directly to the base oftransistor 38. Resistor 43 is so selected as to bias the base oftransistor 38 and as a result the power transistor conducts to supplyenergy from the battery to the load. Upon resumption of the AC source ofpower, the lamps are extinguished due to the positive voltage which isapplied across the resistor 48 resulting in a reverse bias currentthrough this resistor overcoming the forward bias current of resistor49. This reverse bias current turns off transistor 44 which turns offtransistor 41 and in the end turns off transistor 38. Therefore, whenthe AC power is on, the current in line biases the switch off and whenAC power is lost the switch is biased on.

The effect of the bistable mpltivibrator dropout circuit 37 on theoperation of the switch will now be discussed. This circuit basicallycomprises a pair of transistors regeneratively coupled and having azener diode in the feedback pathsThus there is shown a PNP transistor 55having its emitter connected to the output line 25 of the rectifierthrough resistor 56. The emitter is also connected to the common busline31 through resistor 57. The collector of transistor 55 is connectedthrough resistor 58 to the junction 60. Also tied to this junction isthe base of NPN transistor 51 and bleed resistor 59 which in turn isconnected to the common busline 32. The collector of transistor 51 isconnected through zener diode 61 to the base of the transistor 55 atpoint 63 which in turn is .connected through resistor 64 to the busline31. Transistor 51 has its emitter connected directly to the common line32. Damping capacitor 53 is connected across transistor 55 as shown inorder to dampen any transient, sporadic line pulses. This bistablemultivibrator dropout circuit is capable of existing in two stablestates, one referred to as the conductive state and the other as thenonconductive state, and is used to protect the battery fromoverdischarging. The dropout level of this circuit is governed by thecharacteristic voltage of zener diode 61.

The dropout circuit exists in the conductive state whenever the AC powersource is on, or when the power source is off and the battery voltagehas not dropped to the level determined by the characteristic voltage ofthe zener diode in the feedback path of this circuit. Once the batteryvoltage has dropped to the lower potential the bistable circuit switchesto a nonconductive state and turns the switch off thereby extinguishingthe lamp. The bistable multivibrator does not return to the conductivestate until the AC power is restored. Therefore, once the batteryvoltage has dropped to the selected voltage the battery is disconnectedfrom the load and even if its voltage may recover and rise to its highervalue, the dropout circuit will not become conductive and therefore thebasic switch will be unable to turn on the lamp. It is the AC powersource that sets the bistable circuit in a conductive state and itremains in the conductive state so long as the AC power is on or so longas the battery potential remains above the predetermined dropoutpotcntial. However, once the bistable circuit switches to thenonconductive state, the battery is unable along to reset the circuit tothe conductive state.

The bistable circuit is reset to its conductive state by a triggeringpositive current supplied from the rectifier to the emitter oftransistor 55 via resistor 56. This current is sufficient to forwardbias the transistor 55 causing an emitter-base current and consequentlyan emitter-collector current to flow through the transistor. Theemitter-collector current of transistor 55 flowing through resistor 58acts as a base bias on transistor 51. The resistor 59 is a bleedresistor for turning off transistor 51. Since the two transistors areregeneratively connected, their currents rapidly build up to saturation.The small current that is'fed into the common conductor line 32 iseventually returned through the charge controller 35 to the rectifiertransformer 20. The bistable circuit is capable of being reset to itsconductive state by the AC source regardless of the battery potential.This is helped by connecting zener diode 66 and resistor 67 in seriesbetween the rectifier output line 25 and the juncture point 60. Thezener diode 66 is selected so that it will pass current only uponrestoration of the AC power source. It should be noted that bothresistor 56 and the branch line 65 of zener diode 66 and resistor 67serve to reset the bistable multivibrator 37. With the proper choice ofelements, either resistor 56 or the branch line 65 could be used aloneas the resetting mechanism and could reset the bistable circuit upon ACpower restoration regardless of the battery potential. However, in someinstances it has been found best to use the resistor 56 and the branchline 65 in combination to assure reliability and therefore they areillustrated together in the Figure.

Considering now a situation where the AC source has been off for aperiod of time and the battery potential has begun to drop, it will bediscussed how the dropout circuit assumes the nonconductive state andprotects the battery against damage from being overcharged. As pointedout earlier, the choice of the zener diode 61 determines at whatpotential this dropout circuit will assume the nonconductive state sincethe bulk of the potential drop in this circuit is across'the zenerdiode. Thus as the battery potential drops to a low level, the potentialacross resistor 57 is reduced to the point where insufficient current vis supplied to the base of transistor 51 and that transistor ceases toconduct. This in turn rapidly causes transistor 55 to turn off so thatthe dropout circuit assumes the nonconductive state and extinguishes thelamp by causing the switch 36 to turn off. Since the zener diode 61 isin the feedback path of the bistable circuit, it accelerates the actionof turning off the bistable circuit. Since resistor 57 determines thecurrent through transistor 55 and consequently the bias current to thebase of transistor 51, this resistor can be varied to adjust to actualdropout level within a narrow voltage range centered around thepotential determined by zener diode 61.

lt is a property of electric storage batteries such as lead-acidbatteries that after they have been removed from the circuit for aperiod of time, the potential will rise again, Therefore, after thebistable circuit has assumed the nonconductive state, the battery willeventually rise to its higher value. The bistable circuit howeverremains nonconductive since point 63 assumes the potential of thebattery positive terminal and acts as a reverse bias on the base oftransistor 55. Resistor 64 serves as a bleed resistor to prevent thermalrunaway of transistor 55. it is only after the AC power is restored thatsufficient bias is supplied via resistor 56 to the emitter-base junctionof transistor 55 that the bistable circuit reverts to the conductivestate. Also branch line 65 supplied a bias current to the base oftransistor 51 to assure resetting of the bistable circuits. As notedpreviously, either resistor 56 or branch line 65 alone could serve asthe resetting mechanism.

The charge controller circuit 35 is used for monitoring and maintainingthe proper charge on the battery 30. The charge controller has a pair ofdiodes 70, 71 each having its cathode connected to a terminal of thetransformer 20. Each diode has its anode tied to the lead 72 which formspart of a loop having resistor 73, zener diode 74 and resistor 76 andterminating on the common busline 31. Tied into this loop at point 75 isthe base of PNP transistor 77 which has its emitter connected to the busline 31 through resistor 78 and variable resistor 79. The collector oftransistor 77 is tied to common conductor 32 by resistor 80. When the ACpower is on a trickle charge on the battery is maintained throughtrickle resistor 73. The emitter of transistor 77 is connected throughdiode 81 to the emitter of a PNP transistor 82 while the collector oftransistor 77 is connected to the ,base of transistor 82 by resistor 83,

thereby forming a Schmitt-trigger. The collector of transistor 82 isconnected to line 72 through resistors 84 and 85 and the gate of thesilicon controlled rectifier (SCR) 86 is connected to the line joiningresistors 84 and 85. The SCR is positioned in common conductor '32 withits anode connected to the negative terminal of the battery and itscathode connected to line 72 through high rate resistor 88 which governsthe high rate battery charging current.

In operation, a trickle charge is maintained on battery 30 throughresistor 73 and while the battery potential is in a charged condition,theresistor 76 biases the base of transistor 77 to cause that transistorto conduct. When the battery potential drops, the potential drop acrossresistor 76 decreases until a point is reached where the potential atpoint 75 on the base of transistor 77 more nearly approximates thepotential of the emitter of that transistor. This causes transistor 79to cease conducting. Zener diode 74 acts as a reference potential topermit resistor 76 to be the control on transistor 77.

When transistor 77 ceases to conduct, this triggers transistor 82 toconduct and current flows through resistors 84 and 85. The voltage dropacross resistor 85 is applied to the gate of SCR 86 which is triggeredto conduct and high rate charging current is supplied to the battery.Resistor 88 controls the high rate charging current.

When the battery voltage rises, resistor 76 senses the increase inpotential and the base of transistor 77 is biased to cause thattransistor to conduct. Transistor 82 is then turned off which causes SCR86 to cease conducting and the trickle charge is again applied to thebattery 30.

In FlG.'2 there is shown an alternative circuit of this invention havinga delay-on timer connected between the rectifier and the point 102. Thedelay-on timer is useful in those applications where it is desired tokeep the emergency lights lit for a period of time after restoration ofthe AC power source. An example of such an application is where mercuryvapor lamps are used in a building. Such lamps require a warmup periodand therefore it is desirable that the emergency lamps remain on duringthis time. The duration of the time delay is governed by the timeconstant of the series circuit of capacitor 90 and resistor 91 whereinthe capacitor is connected to the output line 25 of the rectifier and tothe common conductor 32 through resistor 91. A PNP transistor is shownat 92 and has its base and collector tied to either side of resistor 91.Just after power restoration, capacitor 90 has zero potential across itand the base of 92 is at the potential of line 25 which in turn causes areverse bias on diode 93 which has its cathode connected to the emitterof transistor 92. A second diode 94 has its anode connected to thebusline 31 and its cathode tied to the junction point 95 which isdirectly connected to the base of NPN transistor 96. The point 95 isconnected to the rectifier output 25 via resistor 97 with the anode ofdiode 93 and one side of capacitor 98 both coupled to the conductivepath shown between point 95 and resistor 97. The emitter of 96 is tiedto the common busline 31.

As shown in H6. 2 an NPN transistor 99 has its collector joined to therectifier output line 25. The emitter of 99 is connected to point 102which in turn is tied to the cathode of Zener diode 66 and to resistors48 and 56. Bias resistor 101 is joined to the collector and the base oftransistor 99, the base of which in turn is connected to the collectorof transistor 99 and to filter capacitor 98. Transistor 99 is an emitterfollower so that when transistor 96 is conducting the emitter oftransistor 99 is at the same potential as busline 31 and therefore nocurrent flows through transistor 99.

The timer circuit works in the following manner: immediately uponresumption of AC power, current is flowing through resistor 97 whichturns on transistor 96 so that transistor 99 is rendered nonconductingand therefore a signal indicating return of AC power is not supplied tothe switch 36 and the dropout circuit 37. As the potential acrosscapacitor 90 builds, the reverse bias on diode 93 lessens until a pointis reached at which transistor 92 conducts. As a result, current is thendrawn through diodes 93 and 94 and away from the base of transistor 96causing that transistor to cease conducting. Bias current is suppliedthrough resistor 101 to the base of transistor 99 and that emitterfollower follows" transistor 96 and conducts when 96 ceases to conduct.Once transistor 99 conducts, the current signalling the switch to turnoff appears across resistor 48 and the switch goes off. If prior 'to ACrestoration the dropout circuit had caused to the switch to go off byassuming its nonconductive state, the dropout circuit would be reset toits conductive state by the current signal.

A modification of the delay-on timer involves substituting a siliconcontrolled rectifier (SCR) for transistor 99 wherein the gate of the SCRwould be tied to the resistor 101 as is the base of transistor 101 andthe anode of the SCR would be connected to point 102 and the cathodetied on line 25.

EXAMPLE I In the following example, there are given the values of thevarious components of the circuit shown in FIG. 1 wherein it is presumedthat a 6-volt battery is used and the rectified poten tial from thestepdown transformer is approximately 10 volts.

Resistors Number Ohms 42 22 43 7.5 46 2.2K 48 LOK 49 LOK 56 4.7K 57 LOK58 LOK 59 LOK 64 l.0K 67 ISO 73 7l.5 76 270 78 33 79 0.50 83 820 84 30085 IX 88 l Transistors Number Designation 38 2N l 557 4| 2N49l9 442N4l25 5| 2N4l23 SS 2N4l25 77 2N4l25 82 2N4l25 Diodes Number Designation23 lN400l 24 lN400l 61 523.7, 1% 66 SZISD, 5% 70 lN47l9R 7l lN47l9R 74SZ87, 5.0, 2% 81 lN4454 All capacitors are 10 mfd., 25 VDC and thesilicon controlled rectifier 86 is a 2N4442. An important feature of thelow-level dropout circuit is the fact that the choice of the Zener diodedetermined at one point the circuit will drop out. Therefore, as shownin the above list of components, the zener diode used in the example hasa potential drop across it of approximately 3 volts and as a result,when the battery potential drops to approximately 4% volts, the bistablemultivibrator will snap into the nonconductive state and can cause thebasic switch to turn ofl. With the choice of another zener diode, thepotential at which the bistable circuit snaps into the nonco nductivestate can be varied. This circuit has been found to be quite reliable inpractice and can be quite useful in emergency power systems.

EXAMPLE II In the following list are shown the values for the componentsused in the delay-on timer shown in FIG. 2 and as incorporated in thebasic circuit of FIG. 1.

Resistors Number Ohms 91 It) Meg. IOI lOK Transistors Number Designation96 ZN l 306 99 ZNSHIIA Diode Number Designation Capacitors NumberDesignation 90 I mfd.

98 0.1 ml'd.

With a delay-on timer circuit using the components listed above, thereis provided a nominal 2 minute delay between restoration of power andextinguishment of the lamp. This is certainly sufficient time to permitmercury lamps or other similar electrical apparatus requiring a periodto warm up to do so.

This invention therefore provides a reliable, and efficient circuit tobe provided in emergency power systems wherein a storage battery is usedto supply energy upon failure of an external supply of electricity.While the description of the preferred embodiment has related inparticular to those applications involving involving emergency lightingsystems, it is understood that this circuit could be used in othersimilar applications. This fully solid-state circuit has severaladvantages along with the fact of reliability including rapid switchingbetween on and off states and providing means for protecting the batteryfrom over discharge as well as providing a means for delaying the signalto turn the switch off when that feature is desired. It is clearly notintended to limit the scope of this invention to the embodimentdescribed in detail herein but rather all reasonable modifications tothis circuit embodied in the spirit of this invention are alsoenvisioned and included.

What I claim is: I. An emergency power supply circuit to be used inconjunction with an AC source of electricity wherein said circuitsupplies energy to a load from a battery when said AC source fails andcomprises in combination:

input means connected to said AC source and having rectifying means, aswitch connected to said rectifying means, to said battery and to saidload and being adapted to complete a current path between said batteryand said load upon loss of AC power, said switch comprising a powertransistor and a pair of transistors coupled together as an amplifier,

a bistable multivibrator dropout connected to said rectifying means, tosaid battery to said switch, said bistable multivibrator being adaptedto protect said battery from over discharge when AC power is off bycausing said switch to disconnect said battery from said load, when thebattery potential decreases to a certain value, and

a charge controller connected to said input means and across saidbattery, said controller being adapted to monitor continually the chargeon said battery and to charge said battery using said AC power as anenergy source when the battery potential drops below a reference value,said charge controller comprising in combination, a pair of transistorsarranged as a trigger comprising a first transistor and a secondtransistor, a current path comprising a resistor and a zener diode andconnected across the terminals of said battery, said first transistorhaving its base connected to said resistor and biased thereby, saidfirst transistor being biased on when said battery is in a charged stateand being reversed biased when said battery is discharged a certainamount, said second transistor being biased on when said firsttransistor is reversed biased and vice versa, a silicon controlledrectifier connected in series with said battery and having its gateconnected through a biasing means to the collector of said secondtransistor, said silicon controlled rectifier being adapted to supply ahigh rate charging current to said battery from said AC source when saidsecond transistor is conducting, and a trickle resistor connected inseries with said battery and adapted to supply a trickle chargingcurrent to said battery from said AC source when said first transistoris conducting.

2. An emergency power supply circuit for use in conjunction with aseparate power source to supply direct current to a load from a batteryupon failure of said separate power source and comprising incombination:

a rectifier input means adapted to be connected to said separate powersource;

an output means adapted to be connected to said load;

a switch connected to said battery and adapted to complete a currentpath between said battery and said output means upon failure of saidseparate power source and to disconnect said battery from said load uponrestoration of said separate power source;

said switch comprising a power transistor and an amplifier comprising apair of transistors;

a bistable multivibrator connected to said switch and to said batteryand adapted to cause said switch to disconnect said battery from saidload when the battery voltage reaches a certain level; SAID BISTABLEMULTIVIBRA- TOR HAVING A VOLTAGE SENSING MEANS IN THE FEEDBACK PATHTHERE OF AND adapted TO EXIST IN A FIRST STATE WHENEVER SAID SEPARATEPOWER SOURCE IS ON AND IN A SECOND STATEE WHEN SAID SEPARATE POWERSOURCE IS OFF AND THE BATTERY VOLTAGE HAS REACHED A CERTAIN LEVEL ANDREMAIN IN SAID SECOND STATE UNTIL SAID SEPARATE POWER SOURCE ISRESTORED: AND,

resetting means to reset said bistable multivibrator from said secondstate to said first state.

3. The circuit ofclaim 2 in which said voltage sensing means in saidbistable multivibrator is a zener diode.

4. The circuit of claim 2 wherein said bistable multivibrator comprisesa first transistor and a second transistor regeneratively coupled toeach other and said resetting means comprises a resistor connecting saidcircuit input means to the emitter of said first transistor.

5. The circuit of claim 2 wherein said bistable multivibrator comprisesa first transistor and a second transistor regeneratively coupled toeach other and said resetting means comprises a series current path of azener diode and a resistor, said current path being connected at one endof said input means and at the other end to the base of said secondtransistor.

6. A circuit for an emergency power supply wherein electrical energy issupplied from a battery to a load upon failure of an AC source of power,said circuit comprising in combination: 7

input means adapted to be connected to said AC power source;

a delay-on timer connected to said input means and having an outputmeans;

a switch connected to said timer output means and to said battery andadapted to connect said battery to said load upon loss of AC power;

a bistable multivibrator dropout means connected to said timer outputmeans and to said battery and to said switch;

said dropout means having a voltage sensing means and adapted to protectsaid battery against over discharge by causing said switch to disconnectsaid battery from said load when said battery is discharged to a certainpotential;

said dropout means adapted to exist in a first state when said AC poweris on and to exist in a second state when said AC power is off and saidbattery has discharged to said certain potential; and, resetting meansconnected to said timer output means and adapted to reset said dropoutmeans from said second state to said first state upon restoration ofsaid AC power;

said timer adapted to delay for a period of time a signal in dicatingrestoration of said AC power from appearing at said timer output.

7. A circuit of claim 6 wherein said timer comprises a series currentpath comprising a capacitor and a resistor, said current path beingconnected to said input means, a first transistor connected to saidcurrent path and adapted to be biased thereby, a second transistorconnected to said input means and to said first transistor and adaptedto be biased first by said input means and second by said firsttransistor, and a third transistor connected to said timer output and tosaid second transistor as an emitter-follower whereby said secondtransistor conducts upon restoration of said AC power and ceases toconduct upon conduction by said first transistor, and said thirdtransistor being nonconductive when said second transistor is conductingand being conductive when said second transistor is nonconducting.

8. A circuit of claim 6 wherein said bistable multivibrator comprises apair of transistors regeneratively coupled to each other and saidvoltage sensing means in the feedback path of said bistablemultivibrator is a zener diode.

9. A circuit of claim 6 wherein said bistable multivibrator drop outcompromises a first transistor and a'second transistor regenerativelycoupled to each other and said resetting means comprises a resistorconnected between said timer output and the emitter of said firsttransistor.

10. A circuit of claim 6 wherein said bistable multivibrator comprises afirst transistor and a second transistor regeneratively coupled to eachother and said resetting means comis substituted for said thirdtransistor and is adapted to be nonconductive when said secondtransistor is conducting and to be triggered into conduction when saidsecond transistor ceases to conduct.

[2. A direct current emergency power supply for use in emergencylighting systems and the like in conjunction with an AC power source anda battery, comprising in combination:

an input adapted to be connected to said AC power source;

a rectifier connected to said input means; i

an output means adapted to be connected to a load, such as a lamp or thelike;

a solid-state switch comprising a power transistor and a pair oftransistors coupled to each other as a darlington amplifier, said switchbeing connected to said battery and to said output means and beingadapted to connect said battery to said output means upon failure ofsaid AC power source;

a solid-state, bistable multivibrator dropout comprising a firsttransistor and a second transistor and having a zener diode in thefeedback path thereof, said dropout being connected to said battery andto said switch and being adapted to exist in one of two states, aconductive state and a nonconductive state, said dropout existing insaid conductive state whenever said AC power source is on and existingin said nonconductive state when said AC power source is off and thepotential of said battery has dropped to a certain level, said dropoutbeing adapted to remain in said nonconductive state until restoration ofsaid AC power source, said dropout being adapted to cause said switch todisconnect said battery from said output means when said batterypotential drops to a certain level and thereby protect said batteryagainst overdischarge;

resetting means for resetting said dropout from said nonconductive stateto said conductive state upon restoration of said AC power source;

a delay-on timer having an input means connected to said rectifier andan output means connected to said resetting means and to said switch,said input means comprising a current path comprising a capacitor, and aresistor connected in series, a first transistor tied to said currentpath and biased thereby, a second transistor adapted to conduct uponrestoration of said AC power and to cease conducting when said firsttransistor conducts, and a third transistor connected as an emitterfollower to said second transistor and adapted to conduct when saidsecond transistor ceases to conduct and thereby supply current to saidtimer output means; and,

a charge controller connected to said input means and across saidbattery, said controller adapted to continually monitor the charge onsaid battery and to supply charging current thereto, said chargecontroller comprising a first transistor and a second transistorregeneratively coupled to each other as a trigger, said trigger havingan input means and an output means, a current path comprising a biasresistor and a reference zener diode, said trigger input means connectedto said bias resistor for biasing said first transistor,

a silicon controlled rectifier connected in series with said battery andhaving its gate connected to said switch output means, said siliconcontrolled rectifier being triggered into conduction upon a current flowin said switch output means.

13. A power supply of claim 12 wherein said resetting means comprises aresistor connected to said timer output and to the emitter of said firsttransistor of said dropout.

14. A power supply of claim 12 wherein said resetting means comprises acurrent path connected to said timer output and to the base of saidsecond transistor of said dropout, said current path comprising aresistor and a zener diode.

1. An emergency power supply circuit to be used in conjunction with anAC source of electricity wherein said circuit supplies energy to a loadfrom a battery when said AC source fails and comprises in combination:input means connected to said AC source and having rectifying means, aswitch connected to said rectifying means, to said battery and to saidload and being adapted to complete a current path between said batteryand said load upon loss of AC power, said switch comprising a powertransistor and a pair of transistors coupled together as an amplifier, abistable multivibrator dropout connected to said rectifying means, tosaid battery to said switch, said bistable multivibrator being adaptedto protect said battery from over discharge when AC power is off bycausing said switch to disconnect said battery from said load, when thebattery potential decreases to a certain value, and a charge controllerconnected to said input means and across said battery, said controllerbeing adapted to monitor continually the charge on said battery and tocharge said battery using said AC power as an energy source when thebattery potential drops below a reference value, said charge controllercomprising in combination, a pair of transistors arranged as a triggercomprising a first transistor and a second transistor, a current pathcomprising a resistor and a zener diode and connected across theterminals of said battery, said first transistor having its baseconnected to said resistor and biased thereby, said first transistorbeing biased on when said battery is in a charged state and beingreversed biased when said battery is discharged a certain amount, saidsecond transistor being biased on when said first transistor is reversedbiased and vice versa, a silicon controlled rectifier connected inseries with said battery and having its gate connected through a biasingmeans to the collector of said second transistor, said siliconcontrolled rectifier being adapted to supply a high rate chargingcurrent to said battery from said AC source when said second transistoris conducting, and a trickle resistor connected in series with saidbattery and adapted to supply a trickle charging current to said batteryfrom said AC source when said first transistor is conducting.
 2. Anemergency power supply circuit for use in conjunction with a separatepower source to supply direct current to a load from a battery uponfailure of said separate power source and comprising in combination: arectifier input means adapted to be connected to said separate powersource; an output means adapted to be connected to said load; a switchconnected to said battery and adapted to complete a current path betweensaid battery and said output means upon failure of said separate powersource and to disconnect said battery from said load upon restoration ofsaid separate power source; said switch comprising a power transistorand an amplifier comprising a pair of transistors; a bistablemultivibrator connected to said switch and to said battery and adaptedto cause said switch to disconnect said battery from said load when thebattery voltage reaches a certain level; SAID BISTABLE MULTIVIBRATORHAVING A VOLTAGE SENSING MEANS IN THE FEEDBACK PATH THERE OF AND adaptedTO EXIST IN A FIRST STATE WHENEVER SAID SEPARATE POWER SOURCE IS ON ANDIN A SECOND STATEE WHEN SAID SEPARATE POWER SOURCE IS OFF AND THEBATTERY VOLTAGE HAS REACHED A CERTAIN LEVEL AND REMAIN IN SAID SECONDSTATE UNTIL SAID SEPARATE POWER SOURCE IS RESTORED: AND, resetting meansto reset said bistable multivibrator from said second state to saidfirst state.
 3. The circuit of claim 2 in which said voltage sensingmeans in said bistable multivibrator is a zener diode.
 4. The circuit ofclaim 2 wherein said bistable multivibrator comprises a first transistorand a second transistor regeneratively coupled to each other and saidresetting means comprises a resistor connecting said circuit input meansto the emitter of said first transistor.
 5. The circuit of claim 2wherein said bistable multivibrator comprises a first transistor and asecond transistor regeneratively coupled to each other and saidresetting means comprises a series current path of a zener diode and aresistor, said current path being connected at one end of said inputmeans and at the other end to the base of said second transistor.
 6. Acircuit for an emergency power supply wherein electrical energy issupplied from a battery to a load upon failure of an AC source of power,said circuit comprising in combination: input means adapted to beconnected to said AC power source; a delay-on timer connected to saidinput means and having an output means; a switch connected to said timeroutput means and to said battery and adapted to connect said battery tosaid load upon loss of AC power; a bistable multivibrator dropout meansconnected to said timer output means and to said battery and to saidswitch; said dropout means having a voltage sensing means and adapted toprotect said battery against over discharge by causing said switch todisconnect said battery from said load when said battery is dischargedto a certain potential; said dropout means adapted to exist in a firststate when said AC power is on and to exist in a second state when saidAC power is off and said battery has discharged to said certainpotential; and, resetting means connected to said timer output means andadapted to reset said dropout means from said second state to said firststate upon restoration of said AC power; said timer adapted to delay fora period of time a signal indicating restoration of said AC power fromappearing at said timer output.
 7. A circuit of claim 6 wherein saidtimer comprises a series current path comprising a capacitor and aresistor, said current path being connected to said input means, a firsttransistor connected to said current path and adapted to be biasedthereby, a second transistor connected to said input means and to saidfirst transistor and adapted to be biased first by said input means andsecond by said first transistor, and a third transistor connected tosaid timer output and to said second transistor as an emitter-followerwhereby said second transistor conducts upon restoration of said ACpower and ceases to conduct upon conduction by said first transistor,and said third transistor being nonconductive when said secondtransistor is conducting and being conductive when said secondtransistor is nonconducting.
 8. A circuit of claim 6 wherein saidbistable multivibrator comprises a pair of transistors regenerativelycoupled to each other and said voltage sensing means in the feedbackpath of said bistable multivibrator is a zener diode.
 9. A circuit ofclaim 6 wherein said bistable multivibrator drop out compromises a firsttransistor and a second transistor regeneratively coupled to each otherand said resetting means comprises a resistor connected between saidtimer output and the emitter of said first transistor.
 10. A circuit ofclaim 6 wherein said bistable multivibrator comprises a first transistorand a second transistor regeneratively coupled to each other and saidresetting means comprises a series current path comprising a zener diodeand a resistor, said current path connected between said timer outputand the base of said second transistor.
 11. A circuit of claim 7 whereina silicon controlled rectifIer is substituted for said third transistorand is adapted to be nonconductive when said second transistor isconducting and to be triggered into conduction when said secondtransistor ceases to conduct.
 12. A direct current emergency powersupply for use in emergency lighting systems and the like in conjunctionwith an AC power source and a battery, comprising in combination: aninput adapted to be connected to said AC power source; a rectifierconnected to said input means; an output means adapted to be connectedto a load, such as a lamp or the like; a solid-state switch comprising apower transistor and a pair of transistors coupled to each other as adarlington amplifier, said switch being connected to said battery and tosaid output means and being adapted to connect said battery to saidoutput means upon failure of said AC power source; a solid-state,bistable multivibrator dropout comprising a first transistor and asecond transistor and having a zener diode in the feedback path thereof,said dropout being connected to said battery and to said switch andbeing adapted to exist in one of two states, a conductive state and anonconductive state, said dropout existing in said conductive statewhenever said AC power source is on and existing in said nonconductivestate when said AC power source is off and the potential of said batteryhas dropped to a certain level, said dropout being adapted to remain insaid nonconductive state until restoration of said AC power source, saiddropout being adapted to cause said switch to disconnect said batteryfrom said output means when said battery potential drops to a certainlevel and thereby protect said battery against overdischarge; resettingmeans for resetting said dropout from said nonconductive state to saidconductive state upon restoration of said AC power source; a delay-ontimer having an input means connected to said rectifier and an outputmeans connected to said resetting means and to said switch, said inputmeans comprising a current path comprising a capacitor, and a resistorconnected in series, a first transistor tied to said current path andbiased thereby, a second transistor adapted to conduct upon restorationof said AC power and to cease conducting when said first transistorconducts, and a third transistor connected as an emitter follower tosaid second transistor and adapted to conduct when said secondtransistor ceases to conduct and thereby supply current to said timeroutput means; and, a charge controller connected to said input means andacross said battery, said controller adapted to continually monitor thecharge on said battery and to supply charging current thereto, saidcharge controller comprising a first transistor and a second transistorregeneratively coupled to each other as a trigger, said trigger havingan input means and an output means, a current path comprising a biasresistor and a reference zener diode, said trigger input means connectedto said bias resistor for biasing said first transistor, a siliconcontrolled rectifier connected in series with said battery and havingits gate connected to said switch output means, said silicon controlledrectifier being triggered into conduction upon a current flow in saidswitch output means.
 13. A power supply of claim 12 wherein saidresetting means comprises a resistor connected to said timer output andto the emitter of said first transistor of said dropout.
 14. A powersupply of claim 12 wherein said resetting means comprises a current pathconnected to said timer output and to the base of said second transistorof said dropout, said current path comprising a resistor and a zenerdiode.